Interactive effects of pH and substrate quality on the fungal-to-bacterial ratio and qCO2 of microbial communities in forest soils

Microbiological and chemical properties of 40 North German spruce (Picea abies [L] K.) and beech (Fagus sylvatica L.) stands at either acidic (∼3.0) or neutral (∼6.0) soil pH (Ah horizon) were investigated to test the effects of pH, substrate type (forest type) and the substrate×pH interactions on soil microbial communities. Soils of each pH set were selected to follow a normal distribution. The percent fungal-to-bacterial respiratory ratio, as determined by the selective inhibition method, was significantly affected by type of forest substrate×pH interactions (P<0.001). Soil pH influenced the percent fungal-to-bacterial ratio to a greater degree than substrate and the effect of pH was more pronounced under beech than under spruce. Mean percent fungal-to-bacterial respiratory ratios (rounded) of 74-to-26 (beech) and 84-to-16 (spruce) were found at high pH (∼6) and ratios of 94-to-6 (beech) and 90-to-10 (spruce) were found at low pH (∼3.0), respectively. The metabolic quotient qCO2 was strongly affected by soil pH (P<0.001). Also, a strong substrate×pH effect was noted (P<0.002). Particularly at low pH the qCO2 was higher under spruce as compared to beech. A relationship emerged between the qCO2 and the fungal-to-bacterial ratio. With increasing fungal presence the qCO2 declined. The Cmic-to-Corg ratio was only affected by pH (p<0.001).